The present invention relates to a coating film adhering to a substrate in a state that transparency of a continuous layer of specific fluorine-containing polymer having heat resistance and non-sticking property is maintained, particularly to a coating film having a thinned layer of fluorine-containing polymer and to a method of forming the thin coating film of fluorine-containing polymer.
Fluorine-containing polymers are excellent particularly in heat resistance, stain-proofing property, non-sticking property, weather resistance, chemical resistance and insulating property and have a low refraction index, and therefore a coating composition and film which are obtained from the fluorine-containing polymer are used or desired to be used for various applications. Particularly perfluoro fluorine-containing resins represented by PTFE, PFA and FEP which have a high heat resistance (not less than 300xc2x0 C.) and a high melting point (not less than 250xc2x0 C.) have the above-mentioned properties at the highest levels, and it can be said that they are the most suitable materials.
On the other hand, on a heat radiation plate and reflection plate of heating apparatuses equipped with a heater such as an electric stove, gas stove and petroleum stove and inner walls of cooking apparatuses such as a range with oven, oven and toaster and fish oven, in order not to release heat (radiation heat) or in order to focus heat on a certain point, materials such as hot-dipped aluminum-coated steel plate having a metal gloss by brightening treatment and stainless steel plate, mirror, etc. having a high reflectance are used. Those materials are desired to be coated with a fluorine-containing polymer having heat resistance, non-sticking property and stain-proofing property not to lower heat reflectance of a substrate. For that purpose, it is desirable to coat a fluorine-containing polymer having the above-mentioned properties on the substrate so that a coating film is transparent and as thin as possible.
However those fluorine-containing polymers have a substantial problem that adhesion to a substrate such as metal is not enough due to excellent non-sticking property thereof to be utilized. Examples of a method of allowing the fluorine-containing polymer to adhere to a metal substrate, etc. are as follows.
1. A method of physically roughening a surface of the substrate by sand blasting or the like.
2. A method of providing a primer layer mainly comprising heat resistant engineering plastic, metal powder, or the like between the substrate and the fluorine-containing polymer.
3. A method of forming fluorine-containing polymer into a film and carrying out chemical treatment such as sodium etching on an adhering surface of the film.
4. A method of using an adhesive.
It is known that those methods are carried out individually or in combination thereof. However only by the method 1, adhesion is insufficient, and peeling arises only by environmental change in temperature and humidity. In the method 2, the primer layer is colored, and therefore only by providing the primer layer, a feature of the substrate such as reflectance is lost. Also in the method 3, coloration of the film occurs, which lowers appearance and features of the substrate, and even if treatment for making a transparent surface can be conducted, adhesion is insufficient and the film is peeled due to thermal deformation when used at high temperature. Accordingly the method 3 must be combined with the methods 1, 2 and 4, and a feature of the substrate is lost as mentioned above. With respect to the method 4, since an adhesive itself has no transparency or its heat resistance is insufficient, the adhesive is colored or whitened when used at high temperature and foaming and peeling arise.
Namely a transparent coating film formed by allowing a fluorine-containing polymer having heat resistance and non-sticking property to adhere directly to a substrate without lowering properties of fluorine has not been obtained.
Further a fluorine-containing polymer having heat resistance and non-sticking property is selected from those being insoluble in a solvent and having a high crystalline melting point, and it is difficult to attain another object of the present invention, namely to allow a transparent continuous layer of those fluorine-containing polymers to adhere to a substrate by making a thickness of the layer extremely thin. In the above-mentioned conventional methods of providing an adhesive layer, a thin coating film has not been obtained.
In view of the facts mentioned above, an object of the present invention is to provide a method of forming a thin coating film of a fluorine-containing polymer having heat resistance and non-sticking property by adhering the fluorine-containing polymer directly to a substrate without lowering a reflectance and light transmissivity of the substrate. Another object is to provide the thin coating film of fluorine-containing polymer which is obtained by applying the fluorine-containing polymer directly to the substrate by the forming method.
The present invention relates to the thin coating film having a thickness of less than 3 xcexcm which comprises a continuous layer of fluorine-containing polymer formed directly on a substrate by adhesion and is characterized in that the fluorine-containing polymer in the coating film has a hydrophilic functional group and a crystalline melting point of the fluorine-containing polymer is not less than 200xc2x0 C.
In that case, it is preferable that a thickness of the coating film comprising the continuous layer of fluorine-containing polymer is not more than 2 xcexcm, more preferably not more than 1 xcexcm.
Also it is preferable that the crystalline melting point of the fluorine-containing polymer in the coating film is not less than 300xc2x0 C.
It is preferable that the hydrophilic functional group is at least one of hydroxyl, carboxyl, salt of carboxylic acid, sulfonic acid group or salt of sulfonic acid.
Also it is preferable that the fluorine-containing polymer having a hydrophilic functional group is a fluorine-containing polymer prepared by copolymerizing (a) 0.05 to 50% by mole of at least one of ethylenic monomers having any functional group selected from hydroxyl, carboxyl, salt of carboxylic acid, sulfonic acid group or salt of sufonic acid with (b) 50 to 99.95% by mole of fluorine-containing ethylenic monomer having no functional group mentioned above.
It is preferable that the ethylenic monomer (a) having functional group is at least one of fluorine-containing ethylenic monomers having any functional group selected from hydroxyl, carboxyl, salt of carboxylic acid, sulfonic acid group or salt of sulfonic acid.
Further the present invention relates to an aqueous dispersion for forming the above-mentioned thin coating film which comprises 0.1 to 70% by weight of fluorine-containing polymer having hydrophilic functional group in the form of fine particles having a particle size of 1 to 200 nm and 30 to 99.9% by weight of water.
Still further the present invention relates to a method of forming the above-mentioned thin coating film, which is characterized in that after the above-mentioned aqueous dispersion is coated on a substrate, sintering is carried out at a temperature of not less than a crystalline melting point of the contained fluorine-containing polymer.